1. Field of the Invention
The present invention relates to piezoelectric devices, and more particularly, relates to a piezoelectric device used for, for example, an oscillator used in a filter for communication and in a clock generator.
2. Description of the Related Art
Conventionally, as piezoelectric resonators for oscillators used in filters for communication and in clock generators, piezoelectric devices formed of piezoelectric ceramics primarily composed of lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) are widely used to employ the shear vibration of the piezoelectric ceramics. In general, the piezoelectric devices have substrates formed of rectangular piezoelectric ceramics and electrodes provided on both main surfaces of the substrate. The electrodes are not formed on the entire surfaces of the substrate, but are formed on parts of the surfaces so that parts of the electrodes oppose each other.
In the piezoelectric device thus described, by appropriately selecting the type of piezoelectric ceramic and the shapes of electrodes, a phenomenon can be realized in which the energy of the piezoelectric vibration of the piezoelectric ceramic is localized at the area between the electrodes which oppose each other, that is, an energy-trapping can be realized. Consequently, a single piezoelectric vibration mode can be obtained, and an effective piezoelectric device as a piezoelectric resonator for an oscillator used in a filter for communication and in a clock generator can be obtained. However, the piezoelectric ceramics have problems in that high-temperature resistance is poor and loss in high-frequency regions is large.
Piezoelectric ceramics having a layered perovskite structure, such as, CaBi4Ti4O15 and PbBi4Ti4O15, have features of high-temperature resistance, small high-frequency loss, and the like, compared to the piezoelectric ceramics primarily composed of lead titanate zirconate or lead titanate. Hence, the piezoelectric ceramics described above are expected to be suitable materials used for piezoelectric resonators which can be used under high temperature conditions or in a high frequency region. However, since the piezoelectric ceramics have strong anisotropic characteristics in the crystal, a high electromechanical coefficient cannot be obtained by a general manufacturing method for piezoelectric ceramics. Accordingly, methods have been proposed in which the c-axis in piezoelectric ceramics having a layered perovskite structure is preferentially oriented in one direction so as to obtain a large electromechanical coefficient. For example, T. Takenaka, et. al., reported that 1.6 times the electromechanical coefficient of piezoelectric ceramics produced by a conventional manufacturing method was obtained in vertical fundamental vibration of a cylindrical oscillator using an orientational ceramic of PbBi4Ti4O15 formed by a hot forging method (J. Appl. Phys., vol. 55. No. 4. 15 (1984)).
In general, in order to obtain a piezoelectric resonator for an oscillator used in a filter for communication and in a clock generator, a single piezoelectric vibration mode having a slight spurious vibration is necessary. In a piezoelectric device using vertical vibration and shear vibration, a single mode is generally obtained by trapping energy using opposing electrodes. However, it has been known that when the Poisson ratio of a piezoelectric ceramic is one-third or less, vertical fundamental vibration cannot trap energy. Poisson ratios of almost all piezoelectric ceramics having a layered perovskite structure, such as CaBi4Ti4O15 and PbBi4T4O15, are one-third or less, so that it is difficult to trap energy.
Concerning higher harmonic vertical vibration, since restriction of a Poisson ratio is not so strict compared to the fundamental wave, it is likely to trap energy; however, in general, the electromechanical coefficient is greatly decreased compared to that of the fundamental wave. Consequently, even though a single vibration mode is obtained, application as a piezoelectric resonator is limited. In contrast, for shear vibration, the electromechanical coefficient is at a level equivalent to that of vertical vibration, and is not restricted by a Poisson ratio.
However, no experiments to trap energy of shear vibration have been performed using a piezoelectric ceramic having a layered perovskite structure in which the c-axis is preferentially oriented in one direction. Even though piezoelectric ceramics having a layered perovskite structure, such as CaBi4Ti4O15 and PbBi4Ti4O15, have high-temperature resistance and little high-frequency loss, which were not provided in conventional piezoelectric materials, no piezoelectric resonator for an oscillator used in a filter for communication and in a clock generator, which can be practically used, has been manufactured as yet.
Accordingly, a major object of the present invention is to provide a piezoelectric device having an electromechanical coefficient of not less than 20%, which can be practically used, by an energy-trapping effect using a piezoelectric ceramic having a layered perovskite structure provided with high-temperature resistance and little high-frequency loss.
A piezoelectric device of the present invention comprises a substrate composed of a piezoelectric ceramic having a layered perovskite structure, and a plurality of electrodes provided at the substrate, in which one crystal axis in the substrate is preferentially oriented, and the substrate is polarized in an approximately orthogonal direction to the direction in which the crystal axis is preferentially oriented, and the plurality of electrodes are formed on surfaces of the substrate which are approximately parallel to the direction in which the crystal axis is preferentially oriented and are approximately parallel to the direction in which the substrate is polarized.
In the piezoelectric device described above, the substrate is preferably formed of a piezoelectric ceramic using a piezoelectric ceramic composition primarily composed of a ceramic composition represented by the formula CaBi4Ti4O15.
When the electrodes are formed on the substrate composed of a piezoelectric ceramic having a layered perovskite structure, and when the direction in which one crystal axis in the substrate is preferentially oriented approximately orthogonal to the direction in which the substrate is polarized, and the electrodes are formed on surfaces of the substrate which are approximately parallel to the direction in which the crystal axis in the substrate is preferentially oriented and are approximately parallel to the direction in which the substrate is polarized, a device having superior high-temperature resistance and little high-frequency loss can be obtained. In addition, the device thus obtained has an electromechanical coefficient which can be practically used.
In the piezoelectric device described above, when the piezoelectric ceramic composition is used, which is primarily composed of a ceramic composition represented by, specifically, the formula CaBi4Ti4O15, superior temperature stability of resonant frequency can be achieved.
The objects described above, other objects, features, and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof with reference to the drawings.